Can someone smarter than me explain this? I recently went to add a custom port to a router on a network so that a customer could use remote desktop on several different computers in his office. When I arrived ports 3389 and 3390 were properly forwarded and could be seen. When I added 3391 it was consistently blocked. I checked the router, firewalls, security software, the switch, the modem, and I plugged directly into the router itself to see if this port was listening. Nobody could register. Finally, I edited the registry on the intended computer so that an alternate port could be used for remote desktop, rebooted the machine and suddenly 3391 is listening. Why couldn't it listen before that? Why did the remote desktop registry edit have to happen first? Shouldn't a port be listening independent of any software?

"Shouldn't a port be listening independent of any software?" No, thqat would open a PC up to an intrusion. Unless there is a specific reason to open a port on a PC it should be blocked - that's what a firewall is for. Michael J

I understand that in a practical sense the port is blocked for security purposes but I'm just trying to understand why it requires software to access that port before the port will register as open. I've always understood that a port is open independent of any one computer if it is forwarded on the router and that did not happen in this case.

If you are forwarding the port from the router to a PC and that PC does not have anything listening on that port then the port would most likely look "dead" to anything trying to send a request. In order for the port to look open there needs to be a response back. That can't happen if there is nothing monitoring traffic on that port. Michael J